FEEDING 267 



cornified papillae, is filled with a peculiar, generally grey, substance 

 — some 4 feet 6 inches thick measured from the palate or along the 

 baleen. This substance, the gum, which consists of non-cornified epidermal 

 cells, is somewhat more delicate than the baleen itself, and, particularly 

 when it is squeezed, looks very much like rubber. In the dermal papillae, 

 which penetrate the gum, van Utrecht discovered arterioles surrounded 

 by networks of venules similar to those found in the dermal papillae of the 

 integument (see Chapter 11). In Chapter 5 we saw that this type of struc- 

 ture may serve for maintaining the body's temperature, and hence its 

 presence in the gum, which comes into constant contact with cold water, 

 is not surprising. 



The gum, like all other epidermal tissue exposed to continuous friction, 

 undergoes constant cell division by which worn-off material is continually 

 replenished from the base layers. Because of this cell division and the con- 

 sequent outward migration of cells, tensions are set up in a specific spot 

 close to the wall of the baleen. These tensions cause cornification of the 

 gum cells. The resulting cornified layer is pushed out with the tubules 

 and the gum, and emerges as the cortical layer of the tubules. The gum 

 itself contributes no further material to the baleen, new material being 

 added exclusively by the cells covering the walls of the baleen where they 

 face the gum, i.e. the cells between the baleen and the special cornified 

 layer which we have just mentioned. The cells of this intercalated layer 

 shift outward with the horn tubules and the cells of the gum, and gradually 

 become cornified. Hence the thickness of the cortical layer is determined 

 by the thickness of this special intercalated layer, which, in turn, is 

 determined by processes that take place in the specific regions shown on 

 Fig. 145. Differences in thickness which may be produced in these regions 

 at any moment are always reflected in the layer, no matter how far it has 

 been pushed out in the course of the years. We emphasize this fact, because 

 the fine structure of the baleen, and particularly of its cortical layers, is an 

 important means of determining a whale's age. We shall return to this 

 question in greater detail in Chapter 14. 



We have seen that the baleen acts like a strainer, and we shall now 

 investigate what happens when a whale feeds. While no one has ever seen 

 what exactly happens when a Mysticete swallows its food, we know that 

 there is a marked difference between the feeding of Right Whales with 

 their very long baleen and of Rorquals with their very short baleen and 

 mouths that can be greatly distended from the bottom. Right Whales 

 seem to swim through thick masses of krill with almost constantly open 

 mouths. The water streams into the mouth and through the openings 

 between the baleen plates while the krill is kept back by the hairy fringe. A 



